By means of calendering of paper, attempts are made to apply an improving effect to smoothness, glaze and surface properties of paper. The calendering temperature has a significant effect on the properties of paper so that, for example, if it is desirable to keep the smoothness that has been achieved invariable, by raising the temperature it is possible to lower the linear load that is used. On the other hand, if it is desirable to keep the linear load unchanged, the smoothness of the paper can be improved by raising the calendering temperature. This is why, in paper machines, paper is hot-calendered, for example, by means of a what is called gradient calender. Thus, today a gradient calender comprises two heatable rolls that form a nip with one another, the paper web being passed through the calendering nip between said rolls. It is a problem of the present solutions that at least one of the heatable rolls that form the calendering nip must be a variable-crown roll in order that the desired uniform linear load could be obtained in the calendering nip. However, making a variable-crown roll into a hot roll is highly expensive, and especially when surface temperatures of an order of 200.degree. C. and higher are required from calender rolls, the limits of the present-day technology constitute an obstacle. In the light of the prior-art knowledge, the service lives of hydraulic fluids and of hydraulic components in general become decisively shorter when the temperature of the fluid exceeds 200.degree. C. At the same time, the cost of operation of the system becomes unduly high. These problems are manifested in particular in connection with a heatable variable-crown roll. It is a second factor which increases the costs significantly that in the prior-art calenders the diameters of the calender rolls must be quite large in order that the construction could be made robust enough.